Constructor Access Rules

Question

if I compile (under g ++) and run the following code, it prints "Foo :: Foo (int)". However, after private instance constructors and assignment operators are closed, it cannot compile with the following error: "error:" Foo :: Foo (const Foo &) is private. "How does it need a copy constructor if it calls the standard constructor at runtime?

#include <iostream>

using namespace std;

struct Foo {
    Foo(int x) {
        cout << __PRETTY_FUNCTION__ << endl;
    }


    Foo(const Foo& f) {
        cout << __PRETTY_FUNCTION__ << endl;
    }

    Foo& operator=(const Foo& f) {
        cout << __PRETTY_FUNCTION__ << endl;
        return *this;
    }
};

int main() {
    Foo f = Foo(3);
}

Answer 1

The copy constructor is used here:

Foo f = Foo(3);

This is equivalent to:

Foo f( Foo(3) );

where the first set of parens re calls the copy constructor call. You can avoid this by saying:

Foo f(3);

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Answer 2

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Foo f = Foo(3);

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Foo f(3);

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